1. Field of the Invention
The present invention relates to improvements in a paint discharge rate control system.
2. Description of the Prior Art
Recently, an automatic painting machine, in general, has been employed to paint automobile bodies and the like. Typical automatic painting machines are disclosed in Japanese Patent Laid-open (Kokai) Nos. 60-61076 and 60-61077.
A known automatic painting machine is shown in FIG. 1 by way of example. This known automatic painting machine comprises an automatic painting machine data processing unit 1, a microcomputer 2, a painting nozzle 3, a mass flow meter 4, a pneumatic regulator 9, an electropneumatic transducer 10, color change valves 14, regulators 15, paint supply tubes 16, a cleaning thinner supply tube 17, a cleaning air supply tube 18, a painting control valve 20, and a paint feed line 21. The discharge rate of the paint is controlled basically by the pneumatic regulator 9 and the electropneumatic transducer 10 on the basis of a discharge signal issued to the microcomputer 2 by the processing unit 1. However, since it is difficult to control the discharge rate accurately only on the basis of the discharge signal, a signal indicates of the mass flow rate detected by the mass flow meter 4, such as a Coriolis-type mass flow meter, is compared by the microcomputer with a predetermined mass flow rate stored beforehand in the microcomputer 2, and then the microcomputer 2 issues a signal representing a correction corresponding to the difference of the detected mass flow rate from the predetermined mass flow rate to the electropneumatic transducer 10 to correct the mass flow rate.
This conventional automatic painting machine thus constituted has the following problems. Since the automatic painting machine has only a single paint feed tube, namely, the paint feed tube 21, paint remaining in the paint feed tube 21 must be blown out through the painting nozzle 3 by opening the air valve 14P and the thinner valve 140 after closing the color change valves 14A to 14N when changing a color paint being used for another. Such an operation for blowing out a color paint is designated as a "push-out operation". However, an enormous amount of paint is wasted in a year by the push-out operation, which has become a significant cost problem in recent years where expensive high-grade paints are used. To utilize the residual paint effectively, there is proposed a method in a push-out operation, in which the color change valve 14 for the color paint being used is closed near the end of the painting operation to use the color paint remaining in the paint feed tube for painting in the final stage of the painting operation.
This proposed method, however, causes the microcomputer 2 and the mass flow meter 4 to detect a false the flow of the paint, which disturbs the control of flow of the paint subject to the push-out operation. That is, when the thinner flows through the mass flow meter 4 during the progression of the push-out operation, the mass flow meter 4 detects the mass flow rate of the thinner and issues a detection signal representing the mass flow rate of the thinner, which is different from that of the paint. Then, the microcomputer makes a decision on the basis of the detected mass flow rate of the thinner, without correcting the detected mass flow rate of the thinner on the basis of the difference in density between the thinner and the paint, that the flow rate of the paint is excessively small, and then issues an erroneous correction signal based on the erroneous decision to the electropneumatic transducer 10. Consequently, the paint is sprayed from the painting nozzle 3 at an excessively high flow rate coating the automobile body with a paint film having an irregular thickness, and thus the quality of the paint coating is deteriorated.
The conventional paint discharge rate control system requires entering of the densities of individual paints into the microcomputer 2 because the density of paint is dependent on the color of paint, the ambient temperature and the ambient humidity. The microcomputer 2 calculates a set mass flow rate by multiplying the density of paint by a set flow rate. However, it is troublesome to change the stored value of the density of paint by color or season.